I'll nominate Ethan Frome by Edith Wharton (1911 Pulitzer). It's a short read set during a winter storm, perfect for December. Available at all the usual public domain places.

Quote:

Set against the bleak winter landscape of New England, Ethan Frome tells the story of a poor farmer, lonely and downtrodden, his wife Zeena, and her cousin, the enchanting Mattie Silver. In the playing out of this short novel's powerful and engrossing drama, Edith Wharton constructed her least characteristic and most celebrated book. In its unyielding and shocking pessimism, its bleak demonstration of tragic waste, it is a masterpiece of psychological and emotional realism.

I am going to nominate a science fiction classic...I, Robot by Isaac Asimov.

Quote:

The three laws of Robotics:
1) A robot may not injure a human being or, through inaction, allow a human being to come to harm
2) A robot must obey orders given to it by human beings except where such orders would conflict with the First Law.
3) A robot must protect its own existence as long as such protection does not conflict with the First or Second Law.

With these three, simple directives, Isaac Asimov changed our perception of robots forever when he formulated the laws governing their behavior. In I, Robot, Asimov chronicles the development of the robot through a series of interlinked stories: from its primitive origins in the present to its ultimate perfection in the not-so-distant future—a future in which humanity itself may be rendered obsolete.

Here are stories of robots gone mad, of mind-read robots, and robots with a sense of humor. Of robot politicians, and robots who secretly run the world—all told with the dramatic blend of science fact and science fiction that has become Asmiov's trademark.

Having looked up the definition of classic on Dictionary.com ("a literary work of the first rank, especially one of demonstrably enduring quality") I second the nomination of The Good Earth. It won the Pulitzer Prize 80 years ago which establishes that it is of first rank. My 91-year-old mother-in-law struggled to finish it in the last days of her life. She said that she had always meant to read it and didn't want to die before finishing it. For me, that established 'enduring quality'.

I thought the hard rule was that it had to be out of copyright in Life + 50 countries?

If that was a rule, that would eliminate a lot of "classics" that are not out of copyright in life+50 countries. I, Robot that I nominated is a classic, but it is not out of copyright in life+50 countries. So that rule may have just been a suggestion.